A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. The pattern may be applied via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle. One particular application of scatterometry is in the measurement of feature asymmetry within a period target. This can be used as a measure of overlay error, for example, but other applications are also known. In angle resolved scatterometers, asymmetry can be measured by comparing opposite parts of the diffraction spectrum (for example comparing the −1st and +1st orders in the diffraction spectrum of a periodic grating. This can be done simply in angle-resolved scatterometry, as is described for example in published patent application US2006066855A1. With reduction of the physical dimensions in lithographic processing, there is demand to reduce the space occupied by targets dedicated to metrology. Image based scatterometry measurements have been devised to allow the use of smaller targets, by taking separate images of the target using −1st and +1st order radiation in turn. Examples of this image based technique are described in published patent applications US20110027704A, US20110043791A and US20120044470.
Demand for further reduction in target size continues, however, and the existing techniques suffer from various constraints that make it difficult to maintain accuracy while reducing the size of the targets. An example of an angularly resolved scatterometer comprising a Solid Immersion Lens (SIL) is disclosed in published patent application US 2009316979 A1. The extreme proximity of the SIL with the target results in a very high effective NA larger than 1. Using a coherent light source with this SIL allows a very small target to be inspected.
To take advantage of the increasing numerical aperture, the gap between the SIL and the target needs to be set and maintained to an optimal value. For example, the gap may be within the range of 10-50 nm to maintain the SIL in effective optical contact with the substrate. SILs are known for use in optical recording. An optical gap measuring method and apparatus for an optical recorder is described for example in patent U.S. Pat. No. 5,953,125 A. The gap in that example is controlled by detecting cross components of polarization in the high numerical aperture element. The cross polarized signal is then recorded by a detector and can be used as an input parameter into a gap control process. A gap control method based on the detection of cross polarized component signal is expected to be a robust method when applied to media such as optical disks where the substrate is uniform (e.g. Aluminum coated grooved substrate with a photosensitive medium). However, for applications involving a wider variety of targets, possibly of unknown structure or composition, known techniques can fail to work with certain samples.
However, when the known techniques for gap measurement are applied to semiconductor metrology some issues may arise. For instance, if a same light source is used for illuminating the target and for generating an air gap control signal, then the requirement for the source power becomes high. If a monochromatic light source is used for generating an air gap control signal, process dependent issues may appear. For example there is a strong possibility that some target layers will exhibit a low to almost zero light reflectivity at a given wavelength. Using a monochromatic light source also creates problems due to speckle noise in the imaging system.
A microscope including a SIL is disclosed in Ghislain et al. Appl. Phys. Lett., Vol. 72, No. 22, 1 Jun. 1998. The gap in this example is controlled by reference to reflected laser light intensity. As the tip approaches the sample, reflections from the SIL and the sample interfere, and the reflected intensity oscillates with a period equal to λ/2. This interference effect is used to bring the sample into the near field of the SIL. In addition, it is mentioned that white light interference between the curved surface of the SIL probe tip and the sample produces a pattern of Newton's rings (intensity oscillates as a function of radial position) that also provide a measure of the tip-sample gap.